The invention is directed to electrophotographic copying equipment and more particularly to an apparatus to pneumaticly dispense a small amount of charge director to a liquid developer reservoir.
In the operation of liquid electrophotographic copying equipment, a liquid developer composition or working dispersion is utilized in the process of producing hard copy on a given substrate using the information depicted on an original document. The working dispersion may comprise, e.g., a carrier liquid such as aliphatic, isomerized hydrocarbons, toner pigment particles dispersed throughout the carrier liquid and a charge director. The working dispersion is stored in a liquid developer reservoir mounted within the copying machine and is used in the electrophotographic copying process as required. Charge director is added to the carrier liquid to increase the conductivity of the working dispersion.
It has been determined, for example, that the addition of one drop of 10% charge director "soybean lecithin" to a liquid developer reservoir containing 800 grams of 1.5% weight by weight dispersion of toner particles within the carrier liquid will increase the conductivity of the liquid developer composition by approximately one picomho/cm. The smallest natural drop of charge director equals approximately 30 mg. which is equivalent to 40 .mu.l. The size of a natural drop depends on the characteristics of the liquid and, therefore, provides good control over the amount of charge director dispensed into the working dispersion reservoir.
Accordingly, in order to take advantage of the control afforded by a natural drop, it is necessary to dispense a very small amount of charge director, e.g. 40 .mu.l, to the reservoir to obtain the desired increase in conductivity of the liquid developer composition. Moreover, due to the nature of charge director, it is preferred that the dispensing mechanism should not have any moving parts within the charge director fluid. Furthermore, a charge director dispensing apparatus must be operable such that there is never any excessive flow of charge director in addition to the desired flow.
It is, therefore, a primary objective of the invention to provide an apparatus which is operable to dispense charge director in volumes as small as that of a single drop (i.e. 30-50 .mu.l) to a liquid developer reservoir without any moving parts within the charge director fluid and without being subject to leakage, pressure or other effects which may result in additional charge director flow into the liquid developer reservoir. Generally, the apparatus of the invention comprises a charge director reservoir including a pressure cell structure submerged below the liquid level of the charge director reservoir. The cell structure is of a small volume. If dispensing of a single drop is desired, the cell volume should be approximately 100 .mu.l, sufficient to hold approximately three drops of charge director. An opening is formed in the cell structure whereby charge director flows naturally from the reservoir through the opening and into the cell structure.
Pursuant to the invention, a first hollow tube is arranged to extend from within the cell structure to the liquid developer reservoir to permit a pressure controlled flow of charge director from the cell structure to the liquid developer reservoir. The tube is configured relative to the cell structure and liquid developer reservoir such that there ordinarily is not any flow between the cell structure and liquid developer reservoir due to gravity or fluid levels. A second hollow tube extends from the cell structure to a source of pulsed air.
Pursuant to a significant feature of the invention, a controlled pulse of air is pumped through the second hollow tube from a source remote from the charge director reservoir to cause a fluid pressure effect upon the charge director within the cell structure resulting in an outward flow of the small amount of charge director through each of the opening of the cell structure and the first hollow tube. The ratio of the diameter of the opening to the diameter of the first hollow tube determines the amount of charge director that will flow out of each of the opening and first hollow tube. In accordance with the invention, the ratio is fixed such that the pulsed air causes the desired volume of charge director to flow through the first hollow tube and into the liquid developer reservoir, and the residual volume of the cell to flow through the opening of the cell. It has been found that the flow ratio is not a function of the duration of the air pulse. It has also been found that there is a minimal duration of the air pulse needed in order to dispense the desired amount of charge director. The minimal duration is determined empirically and is generally a function of the pressure of the air pulse and the length of the first hollow tube. However, the liquid volume thus dispensed into the liquid developer reservoir is almost independent of any increase of the air pulse duration beyond the required minimum.
Pursuant to the preferred embodiment of the invention, an electrical control device is operable to activate the pulsed air pump to dispense the charge director to the liquid developer compositions reservoir. The electrical control device may be selectively activated by a conductivity sensor which monitors the conductivity of the working dispersion within the liquid developer reservoir to determine when charge director is to be added to the reservoir.
Pursuant to another feature of the invention, the liquid developer reservoir is provided with a fluid circulation device to circulate the liquid developer composition within the reservoir. In this manner, the amount of charge director dispensed to the reservoir is readily and evenly dispersed throughout the liquid developer composition due to the circulation of the liquid developer. The thorough mixing of the dispensed amount of charge director and liquid developer composition achieves the desired increase in the conductivity of the liquid developer composition.
Thus, the invention provides a highly advantageous means for accurately dispensing a very small and controlled amount of charge director to a liquid developer reservoir. The volume of the cell structure together with the pressure of the pulsed air and the ratios of the diameters of the opening, first and second tubes may be determined with great accuracy to assuredly provide as small as one drop flow of charge director into the liquid developer reservoir. In addition, the pneumatic dispenser is operable with no moving parts within the charge director reservoir inasmuch as the charge director flow is caused solely by a controlled pulsed air directed to within the cell structure from a remote source.
For a better understanding of the above and other features and advantages of the invention reference should be made to the following detailed description of a preferred embodiment of the invention and to the accompanying drawings.